Ariga et al., Cogent Chemistry (2016), 2: 1134992http://dx.doi.org/10.1080/23312009.2015.1134992

PHYSICAL CHEMISTRY | RESEARCH ARTICLE

Free-radical-induced oxidative degradation of antibacterial drug, methylparaben by permanganate in alkaline medium: A kinetic and mechanistic approachGirish G. Ariga1, Sharanappa T. Nandibewoor1 and Shivamurti A. Chimatadar1*

Abstract: The kinetics of oxidation of an antibacterial drug, methylparaben by permanganate in alkaline medium at a constant ionic strength of 0.80 mol dm−3 was studied spectrophotometrically at 25°C. The stoichiometric ratio between permanganate and methylparaben was found to be 2:1 in alkaline medium. The main products were identified by NMR, IR, and GC–MS spectral studies. The reaction showed first-order kinetics in permanganate, fractional order in methylparaben, and OH− concentrations under the experimental conditions. Ionic strength and dielectric constant did not affect the rate of reaction. The added products did not have any significant effect on the rate of reaction. Based on the rate experimental results, a suitable mechanism is proposed. Investigations at different temperatures allowed the determination of the activation parameters with respect to the slow step of the proposed mechanism. The reaction constants involved in the mechanism were

*Corresponding author: Shivamurti A. Chimatadar, Department of Studies in Chemistry, Karnatak University, Pavate Nagar, Dharwad 580 003, Karnataka, India E-mail: schimatadar@gmail.com

Reviewing editor:Mark Russell StJohn Foreman, Chalmers University of Technology, Sweden

Additional information is available at the end of the article

ABOUT THE AUTHORSGirish G. Ariga passed his M.Sc. degree in Physical Chemistry (2011) from the Karnatak University, Dharwad, Karnataka, India. Currently he is a PhD student in the Department of Chemistry at Karnatak University, Dharwad. His research interests are kinetics, catalysis, and drug protein interaction.

Sharanappa T. Nandibewoor is a professor of Chemistry, P.G. Department of Studies in Chemistry, Karnatak University, Dharwad, Karnataka, India. He obtained his B.Sc. M.Sc. and Ph.D. degrees from Karnatak University, Dharwad. His research interests cover kinetics, catalysis, electrochemical sensors, nano-material applications, and analytical chemistry. He has published more than 350 papers in reputed journals with H-index 22.

Shivamurti A. Chimatadar obtained his B.Sc. M.Sc. and Ph.D. degrees from Karnatak University, Dharwad. Now he is Chairman, P. G. Department of Studies in Chemistry, Karnatak University Dharwad. His research areas are Kinetics, Catalysis, and Drug Protein Interaction. He published more than 100 research articles in National and International journals.

PUBLIC INTEREST STATEMENTThe title of the paper includes kinetic study of oxidation of methylparaben by Mn(VII) in alkaline medium. Methylparaben is used as an antibacterial preservation, cosmetic and anti-fungal agent. The present study deals with free radical induced oxidative degradation and investigate the redox chemistry of [Mn(VII)] in alkaline medium and to compute the thermodynamic quantities of various steps involved in the mechanism to those derived on the basis of kinetic and mechanistic results.

Received: 19 March 2015Accepted: 30 November 2015First Published: 25 January 2016

© 2016 The Author(s). This open access article is distributed under a Creative Commons Attribution (CC-BY) 4.0 license.

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Girish G. Ariga

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evaluated. The results of this study provide fundamental mechanistic parameters and degradation efficiencies of drug by advanced oxidation processes.

Subjects: Bioscience; Food Science & Technology; Physical Sciences

Keywords: permanganate; methylparaben; alkali; kinetics; oxidation

1. IntroductionBioinorganic chemistry constitutes the discipline at the interface of the more classical areas of inor-ganic chemistry and biology. The metal containing compounds has not only been used as biological probes, but also diagnostic and therapeutic pharmaceuticals. The kinetic study also helps us to study the factors like temperature, pressure, substrate concentration, oxidant concentration, composition of the reaction mixture and catalyst which influence the rate of reaction. The permanganate ion (MnO−

4) can oxidize a great variety of substances and it finds extensive applications in organic syn-thesis (Lee, 1980; Lee, Lee, & Brown, 1987; Simandi, Jaky, & Schelly, 1984; Wiberg, 1965). The up-dated literature survey shows that considerable amount of work has been done on the oxidation of organic compounds by potassium permanganate in alkaline media (John & Kee, 1984; Teresa, Maria, & Michal, 1992). The permanganate ion has a tetrahedral geometry with extensive pi-bonding and stable in neutral or slightly alkaline media (Jaky & Simandi, 1981). Manganese (VII) is reduced to Mn(VI) during oxidation processes in basic medium. Permanganate is a versatile oxidizing agent and is used for studying the oxidation kinetics of many organic and inorganic substrates. The mecha-nisms for different organic substrates suggested by various authors are not similar, indicating that a variety of mechanisms are possible, depending upon the nature of the reactive manganese species, the reaction environment, and the nature of the substrate (Cotton & Wilkinson, 1980).

Methylparaben (Methyl 4-hydroxybenzoate), has been widely used as antimicrobial preservative agents in foods, beverages, drugs, anti-fungal agent, and cosmetics for more than 50 years due to their broad antimicrobial spectrum (Soni, Burdock, Greenberg, & Taylor, 2001). Parabens are found naturally in plant sources like grapefruit seed extract (Von, Schluter, Pflegel, Lindequist, & Julich, 1999). All commercially used parabens are synthetically produced, although some are identical to those found in nature. Acute toxicity studies in animals indicate that methylparaben is practically non-toxic by both oral and parenteral routes. The chemical structure of methylparaben is given below:

Methyl 4-hydroxybenzoate

OCH3

O

HO

Metal ions/complexes play a dominant role in synthetic organic chemistry as they have the poten-tial to oxidize a wide variety of organic compounds. Although considerable work has been done on the oxidation of organic compounds by KMnO4, very little attention has been paid to the kinetics of oxidation of esters. In view of pharmaceutical importance of methylparaben, the reaction between methylparaben and KMnO4 in alkali medium is reported in the present work.

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2. Experimental

2.1. Materials and reagentsAll chemicals used were of analytical reagent grade and double distilled water was used throughout the work. Potassium permanganate, methylparaben, (99.5%) was purchased from Himedia Pvt. Ltd. The purity of the drug was checked by comparing its GC–MS spectrum with literature. The perman-ganate stock solution was obtained by dissolving potassium permanganate in double distilled water and standardized by titrating against oxalic acid (Jeffery, Bassett, Mendham, & Denney, 1989). The manganese (VI) solution was made by dissolving potassium manganate (K2MnO4) (AR) in water. KOH (S D Fine-Chem Limited) solution was used to provide required alkalinity and KNO3 solution was used to maintain required ionic strength in the reaction medium.

2.2. Instruments used

(1) For kinetic measurements, Peltier Accessory (temperature control) attached to Varian CARY 50 Bio UV–Visible spectrophotometer (Varian, Victoria-3170, Australia) connected to a rapid ki-netic accessory (HI-TECH SFA-12, UK) was used.

(2) For product analysis, a QP-2010S Shimadzu gas chromatograph mass spectrometer, Nicolet 5700-FT-IR spectrometer (Thermo, USA), 500  MHz 1H NMR spectrophotometer (Bruker, Switzerland), Thermo Finnigan FLASH EA112. CHNS analyses were used.

(3) For pH measurements, ELICO pH meter model Li 120 was used.

2.3. Kinetic studiesThe oxidation of methylparaben by permanganate was followed under pseudo-first-order condi-tions where [methylparaben] was excess over [permanganate] at 25 ± 0.1°C. The reaction was initi-ated by mixing the required quantities of previously thermostatted solutions of methylparaben and permanganate, which also contained constant concentrations of KOH and KNO3 to maintain the re-quired alkalinity and ionic strength. The progress of the reaction was followed by measuring the absorbance of unreacted permanganate in the reaction mixture present in 1 cm cell in a thermostat-ted compartment of a Varian CARY 50 Bio UV–vis-spectrophotometer at 526 nm. It was verified that other constituents of the reaction mixture do not absorb significantly at this wavelength. The appli-cation of Beer’s law to permanganate concentration at 526 nm had been verified and the molar extinction coefficient was found to be ɛ = 2224 ± 100 dm3 mol−1 cm−1. The reaction was followed more than three half lives. The plots were linear over 70% completion of the reaction. The first-order rate constants, kobs were obtained from the plots of log (concentration) vs. time and are reproducible within ±5% (Table 1).

2.4. Stoichiometry and product analysisThe reaction mixtures containing excess concentrations of MnO−

4 to methylparaben in the presence of constant amounts of alkali and ionic strength were kept in an inert atmosphere. The unreacted concentration of Mn(VII) is determined spectophotometrically at 526 nm. Stoichiometry was found to be 2:1, two moles of permanganate consume one mole of methylparaben as shown in Equation (1)

(1)2MnO4- + OH-

HO

O

OCH3+ + MnO4

2-

HO

O

OH+ HC

H

O

.

The reaction mixture in the stoichiometric ratio, under stirred condition, was allowed to react in alkaline media separately for 24 h at room temperature. After completion of the reaction, the reac-tion mixture was neutralized with acid and extracted with ethyl acetate. The obtained products were

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separated by column chromatography and were characterized by NMR, IR, CHN, and GC–MS spectral analysis. The confirmed oxidation products were 4-hydroxybenzoic acid, formaldehyde, and Mn(VI).

Table 1. Effect of variation of permanganate, methylparaben, and OH− concentrations on the oxidation of methylparaben by permanganate in alkaline solution at 25°C and I = 0.80 mol dm−3

[MnO−

4] × 104 (mol dm−3) [MP] × 103 (mol dm−3) [OH−] (mol dm−3) kobs × 103 (s−1)

0.50 5.0 0.3 1.12

1.0 5.0 0.3 1.10

2.0 5.0 0.3 1.11

3.0 5.0 0.3 1.12

4.0 5.0 0.3 1.12

5.0 5.0 0.3 1.10

2.0 0.5 0.3 0.20

2.0 1.0 0.3 0.36

2.0 2.0 0.3 0.61

2.0 3.0 0.3 0.81

2.0 4.0 0.3 0.98

2.0 5.0 0.3 1.11

5.0 5.0 0.1 0.72

5.0 5.0 0.3 1.11

5.0 5.0 0.5 1.44

5.0 5.0 0.8 1.72

5.0 5.0 1.0 1.85

Figure 1. NMR-spectrum of the product 4-hydroxybenzoic acid.

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2.4.1. Spectroscopic Characterization of the Product 4-Hydroxybenzoic acidElemental analysis calculated for C7H6O3 (mol. wt = 138.12); C = 12.0107; H = 1.00794; O = 15.9994. Found: C, 84.07; H, 6.18; O, 48.27. Melting for 4-hydroxybenzoic acid is 213°C (Lit. mp. 214.5°C). 1H NMR (500 MHz, DMSO, at 25°C); δ = 7.780 to 7.757 (d, 2H, Ar–H), δ = 6.819 to 6.791 (d, 2H, Ar–H), 10.206 (s, 1H, –OH), 12.412 (s, 1H, –COOH) ppm (Figure 1). IR spectrum showed an acidic C=O stretch-ing peak at ν = 1681 cm−1, phenolic –OH stretching peak at ν = 1590 cm−1, and carboxylic acid peak v = 3446 cm−1 (Figure 2). The EI spectrum showed an M+ ion peak at 138 amu clearly confirming 4-hydroxybenzoic acid (Figure 3).

The product formaldehyde confirmed by spot test (Feigl, 2012). A drop of test solution is mixed with 2 cm3 sulfuric acid in a test tube, a little chromotropic acid is added, and the test tube was heated for 10 min in a water bath at 60°C. A bright violet color appears indicating the presence of formaldehyde.

Figure 2. IR-spectrum of the product 4-hydroxybenzoic acid.

Figure 3. GC–MS spectrum of 4-hydroxybenzoic acid with its molecular ion peak at 138 amu.

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3. Results

3.1. Reaction ordersThe oxidation of methylparaben by potassium permanganate in alkaline medium, proceed with measurable rates. Hence, the reaction orders have been determined from the slopes of log kobs vs. log (concentration) plots by varying the concentrations of methylparaben and alkali in turn while keeping others constant.

3.2. Effect of permanganate concentrationPermanganate concentration was varied in the range of 0.50 × 10−4 to 5.0 × 10−4 mol dm−3 and the linearity of the plots of log (concentration) vs. time up to 70% completion of the reaction indicates a reaction order of unity in [MnO−

4]. This is also confirmed by varying of [MnO−

4], which did not result in any change in the pseudo-first-order rate constants (Table 1).

3.3. Effect of methylparaben concentrationThe effect of methylparaben was studied in the range of 0.50 × 10−3 to 5.0 × 10−3 mol dm−3 at 25°C and at constant concentrations of MnO−

4, OH− and ionic strength. The rate of the reaction increased with increase in concentration of methylparaben (Table 1). The order with respect to concentration of MP was obtained from the plot of log kobs vs. log [MP] and was found to be less than unit order (0.76).

3.4. Effect of alkali concentrationThe effect of alkali concentration was studied in the range of 0.10 to 1.0 mol dm−3 at 25°C and at constant concentrations of MnO−

4, MP, and ionic strength. The rate of the reaction increased with an increase in the concentration of alkali concentration (Table 1). The order with respect to OH− concen-tration was obtained from the plot of log kobs vs. log [OH−], and was found to be less than unit order (0.41).

3.5. Effect of ionic strength (I) and dielectric constantThe effect of ionic strength (I) was studied by varying the concentration of KNO3 in the range of 0.1–1.0 at 25°C and at constant concentrations of MnO−

4, MP, and OH−. The dielectric constant of the medium (D) was studied by varying the t-butyl alcohol-water percentage (v/v) in the range of 0–25%. The ionic strength and dielectric constant did not have any significant effect on the rate of reaction.

3.6. Effect of initially added productsThe initially added products, p-hydroxy benzoic acid and Mn(VI) (as K2MnO4) did not have any signifi-cant effect on the rate of reaction.

3.7. Polymerization studyTo the reaction mixture a known quantity of CH2=CHCN scavenger (Moelwyn-Hughes, 1947), had been added initially, was kept in an inert atmosphere for 2 h. On diluting the reaction mixture with methanol, precipitate appears in the reaction mixture, indicating the presence of free radical in the reaction.

The observed experimental results lead to the following rate law:

3.8. Effect of temperature (T)The kinetics was studied at four different temperatures, 15, 25, 35, and 45°C, under varying concen-trations of methylparaben and alkali, keeping other conditions constant. The rate increased with increase in temperature. The rate constant (k) of the slow step of Scheme 1 were obtained from the slopes and intercepts of 1/kobs vs. 1/[MP], (Figure 4(a)) and 1/kobs vs. 1/[OH−], (Figure 4(b)) plots at four

Rate = k[MnO−

4 ]1.0[MEP]0.76[OH−

]0.41

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Figure 4. Verification of rate law (6), in the form of an Equation (7), for the oxidation of methylparaben by permanganate in alkaline medium.

(b) 1/ kobs versus 1/ [OH-]

(a) 1/ kobs versus 1/ [MP]

0

2

4

6

8

10

0.0 0.5 1.0 1.5 2.0 2.5

15° C

35° C

25° C

45° C

1/ [MP] dm3 mol-1

1/k

obs×

10-3

(s)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 2 4 6 8 10 12

1/k

obs

×10

-3(s

)

1/[OH-] dm3 mol-1

15° C

35° C

25° C

45° C

Table 2. Activation parameters and thermodynamic quantities for the oxidation of methylparaben by permanganate in aqueous basic medium with respect to the slow step of Scheme 1

(A) Effect of temperature and activation parameters

Temperature (K) k × 103 (dm3 mol−1 s−1) Parameters Values288 1.28 Ea (kJ mol−1) 45 ± 3

298 2.29 ∆H≠ (kJ mol−1) 42 ± 3

308 4.15 ∆S≠ (J K−1 mol−1) −154 ± 5

318 7.44 ∆G≠ (kJ mol−1) 88 ± 4

log A 5.2 ± 0.02

(B) Effect of temperature on K1 and K2

Temperature (K) K1 × 10−3 (dm3 mol−1) K2 × 10−3 (dm3 mol−1)288 7.99 1.27

298 6.43 1.45

308 4.39 1.97

318 3.13 2.63

(C) Thermodynamic quantities for first (K1) and second (K2) equilibrium steps of Scheme 1

Thermodynamic quantities K1 K2

∆H (kJ mol−1) −24 19

∆S (J K−1 mol−1) −86 125

∆G (kJ mol−1) 1.7 −19

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different temperatures (Table 2). The energy of activation corresponding to these rate constants was evaluated from the Arrhenius plot of log k vs. 1/T and from which other activation parameters were obtained (Table 2).

4. DiscussionThe reaction between permanganate and methylparaben in alkaline medium has the stoichiometry 2:1 (MnO−

4:MP) with a unit order dependence on [MnO−

4] and less than unit order in [MP], and [OH−]. No effects of added products were observed. Under these prevailing experimental conditions, the further reduction of MnO2−4 might be stopped (Sukalyan, Sabita, & Bijay, 2009). The active species of MnO−

4, [MnO4.OH]2−, reacts with methylparaben in the second equilibrium step to form a complex, that then decomposes in a slow step to form of methylparaben cation free radical intermediate. In the fast step this free radical intermediate react with alkali to form the product, p-hydroxybenzoic acid, formaldehyde, MnO2−4 with liberation of H˙. Such type of formation of free radicals is evidenced (Laszlo, Erzsebet, Dajkaa, D’Angelantoniob, & Salvatore, 2001; Stephen, Thomas, William, & Julie, 2007; Wolf & Chrita, 1999) by pulse radiolysis work on similar compounds. The attacks of hydroxyl radical in pulse radiolysis (Wolf & Chrita, 1999) studies are different from MnO−

4 ion of methylpara-ben. The products which are confirmed by NMR, GC–MS, and IR studies indicate that phenolic group of the benzene do not involve in the oxidation. In the further fast step another mole of [MnO4.OH]2− reacts with hydrogen radical to form reduced product of Mn(VI) and H2O. The mechanism satisfies the stoichiometric observation. A reasonable mechanism is proposed in Scheme 1.

Scheme 1. Mechanism for the oxidation of methylparaben by permanganate in aqueous alkaline medium.

MnO4- + OH- [MnO4.OH]2-

K1

K2

[MnO4.OH]2- +

HO

O

OCH3 Complex

kComplex

slow

HO

C

O

OCH3+

Mn(VI)+

HO

COH

O

+ Mn(VI)+

HO

C

O

OCH3+

fast+ OH-

HC O

H

H +

[MnO4.OH]2- +fast

H Mn(VI) + H2O

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Detailed mechanism of Scheme 1.

MnO4- + OH- [MnO4.OH]2-

K1

K2

[MnO4.OH]2- +

HO

O

HO

CO

CH

O H

H

MnO

O

O-O-

OH-

OCH3

+

HO

CO

CH

O H

H

MnO

O

O-O-

k

HO

CO

CH

H

H

O

+

+ MnO42-

slow

HO

CO

CH

H

H

O

+ OH- fast

HO

CO

CH

H

H

O-

OH

++

HO

COH

HC

O

H

O

HO

CO

CH

H

H

O

OH

-

+

+fast

H+.

[MnO4.OH]2- + MnO42-fast

+ H2OH.

In alkaline medium permanganate(VII) undergo oxidation to form Mn(VI) (Timmanagoudar, Hiremath, & Nandibewoor, 1997). During this study, color of the solution changes from violet to blue, and then to green. The spectrum of green solution obtained during the oxidation of methylparaben by permanganate is identical to that of MnO2−4 . It is probable that blue color originated from the vio-let of permanganate and green from manganate excluding the accumulation of hypomanganate. The spectral changes during the reaction are shown in Figure 5. It is evident from the figure that the concentration of MnO−

4 decreases at 526 nm and increases at 603 and 453 nm, 2 isobestic points are observed at 603 and 453 nm, due to Mn(VI) (Simandi, Jaky, Savage, & Schelly, 1985). The results sug-gest the formation of a complex (Manmeet, Faqeer, & Zaheer, 2007) between the methylparaben and permanganate.

From the above mechanism the following rate law (6) can be derived as follows:

Rate =−d[MnO−

4 ]

dt= k K1 K2[MnO

−

4 ]f [MP]f [OH−]f

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The total [MnO−

4] can be written as

where t and f stands for total and free concentration of manganese (VII).

Similarly, total [OH−] can be calculated as,

In view of the low concentration of MnO−

4 and MP used in the experiment, the term K1[OH] and 1 + K1K2 [MnO

−

4] [MP] can be neglected in comparison with unity

Similarly,

Substituting Equations (3), (4), and (5) in Equation (2) and omitting the subscripts we get,

or

[MnO−

4 ]t = [MnO−

4 ]f + [MnO4 ⋅ OH−] + [Complex] = [MnO−

4 ]f + K1[MnO−

4 ]f [OH−]f

+ K1 K2[MnO−

4 ][OH−][MP] = [MnO−

4 ]f {1 + K1[OH−] + K1 K2[OH

−][MP]}

(3)[MnO−

4 ]t =[MnO−

4 ]t

1 + K1[OH−] + K1 K2[OH

−][MP]

[OH−]t = [OH−

]f + [MnO4 ⋅ OH]2−

+ [Complex]

[OH−]f =

[OH−]t

1 + K1[OH−] + K1 K2[MnO

−

4 ][MP]

(4)[OH−]f = [OH−

]t

(5)[MP]f = [MP]t

(6)Rate =−d[MnO−

4 ]

dt=

k K1 K2[OH−][MP]

1 + K1[OH−] + K1 K2[MnO

−

4 ][MP]

(6)

Rate

[MnO−

4 ]= kobs =

k K1 K2[MnO−

4 ][OH−][MP]

1 + K1[OH−] + K1 K2[MnO

−

4 ][MP]

Figure 5. Spectral changes during the oxidation of methylparaben by permanganate in alkaline solution at 25°C.

Notes: Conditions [Mno4

−] = 2.0 × 10−4, [MP] = 5.0 × 10−3, [OH−] = 0.3, and I = 0.80 mol dm−3.

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The rate law (6), can be rearranged to Equation (7), which is suitable for verification.

According to Equation 7, plots of 1/kobs vs. 1/[MP] and 1/kobs vs. 1/[OH−] should be linear and are found to be so (Figure 4(a and b)). The formation of the complex was also proved kinetically by the non-zero intercept of the plot of 1/kobs vs. 1/[MP] (Michaelis–Menten plot). The value of K1 (6.43 × 10−1) is in agreement with the literature (Naik, Chimatadar, & Nandibewoor, 2009). The thermodynamic quantities for the first and second equilibrium steps of Scheme 1 can be evaluated as follows. The [MP] and [OH−] (as in Table 1) were varied at four different temperatures. From the slopes and inter-cepts of plots, 1/[MP] vs. 1/kobs and 1/[OH−] vs. 1/kobs, the values of K1, K2, and k were calculated at different temperatures and the values are given in Table 2. The obtained kinetic constants k, K1 and K2 are in good agreement with the literature value (Hegde, Shetti, & Nandibewoor, 2009; Naik et al., 2009; Panari & Nandibewoor, 1999; Veeresh, Lamani, & Nandibewoor, 2009) given in SI Table 2. van’t Hoff’s plots were made for variation of K1 and K2 with temperature (log K1 vs. 1/T and log K2 vs. 1/T). The enthalpy of reaction ΔH, the entropy of reaction ΔS, and free energy of reaction ΔG, were calcu-lated. These values are given in Table 2. The large negative values of ∆S≠ (−154 JK−1 mol−1) suggest the formation of a rigid transition state which is more ordered than the reactants with reduction of the degrees of freedom of the molecules involved (Rafat, 2002). The energy of activation, Ea, en-thalpy of activation, and higher rate constant for the slow step indicate that the oxidation presum-ably occurs via an inner-sphere mechanism. This conclusion is supported by earlier observation (Reuben, Simoyi, Irving, & Kenneth, 1986). The effect of ionic strength and dielectric constant of the medium on the rate explain qualitatively the reaction between neutral and charged species, as seen in the rate-determining step of Scheme 1.

4.1. Chemical oxygen demand studyCOD is the measure of the amount of oxygen required for complete oxidation of the organic matter present in a sample of reaction mixture. Oxygen demand is determined by measuring the amount of oxidant consumed using titrimetric methods (Wayne, 1997). The objective of this study is to investi-gate the chemical oxygen demand (COD) degradation kinetics (Lucjan, Marzenna, & Artur, 2010). Reaction mixtures were prepared by the double distilled water, which is engaged to determine the COD in present work {[MnO−

4] = 2 × 10−4, [MP] = 5 × 10−3, [OH] = 0.3, I = 0.8 mol dm−3}. The first order plot of log [COD] vs. time is shown in SI Figure 1 (SI Table 1) and observed rate constant is 6.88 × 10−3 h−1.

5. ConclusionsThe most interesting feature of this study is the reactivity of methylparaben with MnO−

4 in aqueous alkaline medium, [MnO4·OH]2− is considered as active species. A free radical mechanism is proposed. Rate constant of slow step and other equilibrium constants of different steps, involved in the mecha-nism were evaluated and activation parameters with respect to slow step of reaction were also calculated. The overall mechanistic sequence described here is consistent with product studies, mechanistic and kinetic studies.

(7)1

kobs=

1

k K1 K2[OH−][MP]

+1

kK2[MP]+1

k

Supplementary materialSupplementary material for this article can be accessed here http://dx.doi.org/10.1080/23312009.2015.1134992.

AcknowledgmentThe authors thank to Indian Institute of Science, Bangalore for NMR and CHN analysis./Sch/UGC-UPE/2013/14/1114

FundingGirish G. Ariga thanks Karnatak University, Dharwad for the research fellowship [KU/Sch/UGC-UPE/2013/14/1114] under the UGC-UPE program (2013–16).

Author detailsGirish G. Ariga1

E-mail: girishariga@gmail.comSharanappa T. Nandibewoor1

E-mail: stnandibewoor@yahoo.comShivamurti A. Chimatadar1

E-mail: schimatadar@gmail.comORCID ID: http://orcid.org/0000-0001-5382-73171 Department of Studies in Chemistry, Karnatak University,

Pavate Nagar, Dharwad 580 003, Karnataka, India.

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Citation informationCite this article as: Free-radical-induced oxidative degradation of antibacterial drug, methylparaben by permanganate in alkaline medium: A kinetic and mechanistic approach, Girish G. Ariga, Sharanappa T. Nandibewoor & Shivamurti A. Chimatadar, Cogent Chemistry (2016), 2: 1134992.

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